Joints in General

Joints are concentrated nexuses in the pathway (fascia ~ sinew channels ~ tendons ~ joints ~ bone marrow) that's a big part of qi gong cultivation.  Lots of layers come together at the joints; things tend to bind up there, and conversely they're a major key to releasing and integrating.  In the early part of any good qi gong routine, you should warm-up all of the major joints.

I'm recently a big fan of ankle rotations.

Earth-connection, Layers

To develop earth-connection you have to develop the legs, and the feet-ankles are the place of physical connection to the earth.  If you spend some time rotating the ankles, this works the muscles, tendons, and into the ligament-joint-bone connection (the joints are the gateways into the marrow)... mixing the blood, various fluids, and energetics into all of those denser layers.  The ankle's structure is especially amenable to this sort of nourishment-into-denser-layer blending work.  (See picture.)

Ankle rotations provide a basic connection of earth all the way through the layers of the body, starting from where you physically touch earth.  This sets a good precedent: the integration tends to cascade upward through the rest of the body.

Standing, Free Foot

A common warm-up is ankle rotation: You stand on one foot, and rotate the other foot's ankle in a circular motion, first one way and then the other way.  Then switch feet.

One variation is to do ankle rotations while what'd otherwise be idle lounging.  Sitting on a chair, couch, back-porching, even lying down (this is an exercise that you can do under the covers while you are sleeping in!)...  Rotate the ankles for extended periods, in various patterns, in rhythmic patterns to music if you like.  With some time and variety spent on this, you'll find that ankle rotations really promote circulation through the denser tissues of the ankle and foot.  Take this well beyond warm-up and go for deep tissue change.  If you haven't tried this before, I highly recommend taking it up frequently (several times a day) for at least week or so, at some point.

Standing, Body-Weight

The planted foot (or both feet) gets rotated, with body-weight.  Stand on the ball of your foot, and move the heel in a circle.

Once physical-substance warm-up is achieved, subtler work (such as focusing on Kidney-1, "bubbling spring", which is a primary point through which to develop earth connection) becomes much more effective.

Variation: Wrists Synchronous with Ankles

Another variation on this is to rotate wrist/s, synchronous with ankle/s.  Speed is not the issue, here.  It'll probably feel awkward the first several times; just do it as slowly as is comfortable to keep ankle/s, wrist/s moving together.

With the small addition of synchronous wrist/s movement, the benefits of the ankle/s-rotation-exercise are improved immensely.  There is something about moving distal ends at once that tends to activate an entire layer of the body.

Thorough Warm-up

Ankle rotations should be done until 

        movement is fluent in both directions, and

        the ankles are warm (blood and qi flow through the layers).

Chinese Medicine

For those of you that are into Chinese Medicine, the yuan (source) points of the 12 meridians are located at & near the wrists and ankles.  Also, a bunch of kidney points (and a variety of others: bladder, gall-bladder, spleen) are located around the ankle.  (Translate that as, "ankle rotations connect to energetics that are very helpful for jing cultivation".)

Ankle rotations are one of those simple things that do a lot.

All of the Major Joints

Actually, within the beginning of any qi gong routine, you should warm-up all of the major joints: ankles, wrists, knees, elbows, shoulders, hips, spine (hopefully including some exercises that focus on various sections of spine, as well as whole-spine warm-up/s - perhaps in a variety of ways).  The neck resonates with the sexual center, so thorough warm-up of the cervical vertebrae particularly benefits jing gong.  Neck exercises: Neck isometrics and Scott Sonnon's Intu-Flow program.

Hip Rotations

The Three Amigos of rooting.